In a known fluidized bed apparatus of this kind (DE-OS No. 25 51 578, FIG. 5) the rotor comprises a four-blade impeller disposed directly above the foraminous tray. The individual blades each have a deflector face which rises toward the rear from a front lower edge, as seen in the direction of rotation, disposed a short distance above the foraminous tray, a rear wall which extends down from the upper end of the deflector face, and a blade bottom which interconnects the lower edges of the deflector face and the rear wall. The interior of each blade thus defined communicates with a hollow shaft of the rotor, which shaft is supported below the bottom and connected to the pressure end of a blower. The rear walls of the blades are perforated to permit the passage of drying air supplied by the blower into the treatment chamber above the foraminous tray. Directly below the foraminous tray there is a supply air chamber which is connectable to the pressure end of another blower so that additional drying air may flow upwards through the foraminous tray and between the blades of the rotor into the treatment chamber. The rate of flow is so adjustable that granular or pilular material in the treatment chamber will form a fluidized bed. A nozzle arranged above the fluidized bed or a plurality of nozzles installed in the blades may be used to spray liquid into the fluidized bed, for instance, for varnishing the material being treated.
With this known fluidized bed apparatus a relatively low flow rate, as compared with previously known means, is sufficient to dry granular material of the most varied kinds. Yet there are sensitive granular or pilular materials which dislike the direct action of the rotor blades such as seeds surrounded by peat or nutritive agents. On the other hand, not all types of material are suitable for treatment in a fluidized bed apparatus without a rotor since they may tend to form channels when a gas stream flows from the bottom through the foraminous tray and is uniformly carried through the material. The channel formation has the result that the gas quickly escapes from the top of the channels, leaving a major part of the material untouched. In that event a fluidized or turbulent bed in which all of the material is subjected to the turbulence cannot be obtained.
In another known fluidized bed apparatus (U.S. Pat. No. 3,849,900) in which likewise a foraminous tray separates a treatment chamber from an air chamber disposed underneath, a rotor including a circular orifice plate is disposed directly below the foraminous tray as a means to prevent the formation of channels. The orifice plate is provided with breakthroughs distributed either evenly or unevenly to let drying gas flow from the air chamber into the treatment chamber only through these breakthroughs and the foraminous tray disposed above the same. Thus the entire rate of flow of the gas is concentrated in one or more gas streams circulating in the treatment chamber in accordance with the speed of rotation of the rotor so that they are always forced to find new paths through the material. Therefore, no stationary channels can be established. However, the utilization of the heat capacity of the gas rising in more or less sharp streams through the material is far from complete. As is usual in fluidized bed apparatus, a filter is arranged above the treatment chamber and must be cleaned from time to time by vibration to remove the dust collected. During such vibration the dust falls into the treatment chamber where it may agglomerate undesirably with the material being treated so that the resulting product may be a granular material of very uneven size.